Are only children to blame for the obesity crisis?

Danish researchers say the rising numbers of children who have no siblings… (Justin Sullivan / Getty…)

A new study suggests an intriguing explanation for the rise in obesity rates — the growing number of only children.

Researchers from Denmark analyzed health records of more than 29,000 Danish schoolchildren and found that boys and girls without brothers or sisters were 44% more likely to be obese than kids with siblings. They also looked at the records of several hundred young men who had registered for the draft and calculated that those who were only children were 76% more likely to be obese than their counterparts with siblings.

The results, published online Wednesday in the journal PLOS ONE, are in line with other research. For instance, a study published last year in the journal Nutrition and Diabetes found a 52% increased risk of obesity among only children from throughout Europe.

Among the school kids, obesity was defined as having a BMI in the top 5% of all students when they were 13 years old. The draft registrants (who ranged in age from 18 to 26, with a median age of 19) were considered obese if their BMI was at least 31. (Most studies use a cut-off of 30, but the researchers said they didn’t think the difference changed their results.)

Why would being an only child cause someone to weigh more? Back in the 1940s, the psychoanalyst and eating disorder expert Hilde Bruch blamed ambivalent mothers who overfed their singletons.

More recently, scientists have looked to biology for explanations. One theory — known as the Developmental Origins Hypothesis — is that conditions in the womb are different for the first occupant than for subsequent occupants. But if that were the case, then first-born children with younger siblings should be just as likely to be obese as only children. But they aren’t — in the new study, first-borns were no more likely to be obese than children born later in the birth order. More research is needed, the study authors wrote.

The research team also found that youngest siblings were significantly more likely to be obese. Among the schoolchildren, those who were born last were 33% more likely to be obese than other kids with siblings. (Only children were excluded from this part of the analysis.) Among the young adults who registered for the draft, being last-born was associated with a 32% increased risk of obesity.

The researchers speculated that the higher risk to last-born children might be explained in part by the fact that mothers are older and might weigh more with each successive pregnancy. But if that were the case, they would expect to find that second-born children were more likely to be obese than first-born children, and that third-borns would be more likely to be obese than second-borns. But neither of those things turned out to be true.

“It has previously been observed that last-born children have psychological characteristics like those of only children,” the study authors wrote. “Whether this influences their obesity risk is not known.”

What is known is that only children are becoming more common. In Copenhagen, 16.4% of children had no siblings in the years 1952 to 1970. That figure rose to 18.3% in the period between 1971-1989.

“If only children are at increased risk of obesity, and if not due to their first-born status, they may be an important at-risk group for targeted prevention,” the researchers wrote.